Pad type bearings used in cryogenic (i.e., extremely low temperature) applications are subject to an unique set of operating conditions. The bearings must operate in temperatures in the range of -200.degree. to -300.degree. F. and the fluid used typically has a very low viscosity. As a result, the design of bearings for cryogenic applications such as pumps for use in space has proven difficult.
Known hydrostatic bearings include the type in which a pad is able to move or "float" within limits between the two relatively moving parts of the bearing. In this type of bearing, the pad has, in the past, been accommodated in a chamber or pocket in one of the two relatively moving parts, and, in operation is urged toward the other moving part by fluid pressure acting between the base of the chamber or pocket and the inside face of the pad. For acceptable operation of hydrostatic bearings, it has been found to be appropriate for each bearing pocket to be supplied from a separate pump, or for a shared pump to be incorporated for all or a group of bearing pockets, in which case each individual bearing pocket is preceded by a hydraulic resistance whereby a fall in the pressure in one bearing pocket does not adversely affect the feed to the other bearing pockets fed from the same pump. Since there is a passage through the pad and therefore the fluid pressure at the bearing surface also acts on the outside face of the pad, it is necessary to arrange that the effective area of the inside face is greater than that of the outer face, so that there is a differential force urging the pad outwardly when the hydrostatic bearing is operative.
There have been attempts to use conventional hydrostatic bearings in cryogenic applications. Such conventional hydrostatic bearings typically include separate pads which move by sliding with respect to a base. The relative sliding of these pieces leads to fretting at the pivot location. Moreover, because of this sliding, it is very difficult to prevent leakage in conventional hydrostatic bearings with a contact seal. This is especially true in applications such as cryogenic applications, involving the use of a low viscosity hydrostatic fluid.
In a more general sense, the present invention broadly relates to movable pad bearings. As such, applicants' previous bearing designs form a starting point for the present invention. In this sense, the present invention relates to hydrodynamic bearings that are also sometimes known as movable pad bearings and methods of making the same. Generally these bearings are mounted in such a way that they can move to permit the formation of a wedge-shaped film of lubricant between the relatively moving parts. Since excess fluid causes undesirable friction and power losses, the fluid thickness is preferably just enough to support the maximum load. This is true when the formation of the wedge is optimized. Essentially the pad displaces with a pivoting or a swing-type motion about a center located in front of the pad surface, and bearing friction tends to open the wedge. When the formation of the wedge is optimized, the wedge extends across the entire pad face. Moreover, the wedge is formed at the lowest speed possible, ideally as soon as the shaft begins to rotate.
The so-called tilt-pad radial bearing is by far the most commonly-prescribed design for machines requiring maximum rotordynamic stability because of its exceptional stability characteristics. Consequently, it has become the standard by which many other radial bearings are measured when seeking a highly stable bearing design. The tilt-pad bearing's popularity is evidenced by the large number of applications found in industry, both as original equipment, and as aftermarket replacements. Applications range from small high-speed machines such as turbochargers and compressors, to very large equipment such as steam turbines and generators. The high rotordynamic stability comes from the reduction of cross-coupled stiffness that occurs when pads are free to tilt about their individual pivot points. This attenuates the destabilizing tangential oil film forces that can induce catastrophic subsynchronous vibration in machines equipped with conventional fixed-geometry bearings. Since so many machines are susceptible to this type of bearing-induced instability, there is a large demand for quality tilt-pad bearings.
Because of its many moving parts and manufacturing tolerances, the tilt-pad design is also the most complex and difficult to manufacture of all journal bearing designs. The design complexity is evident in the number of highly-machined parts required to make up the bearing. Clearance tolerances are additive in the built-up assembly of shell, pivots, and pads, requiring a high degree of manufacturing accuracy to yield acceptable radial shaft clearances. Pad pivot friction under high radial load can also lead to premature wear, or even fatigue failure, which can enlarge clearances and increase rotordynamic unbalance response. All of these requirements combine to make the tilt-pad bearing one which demands maximum attention to design, manufacturing, and materials.
The need for close tolerances manifests itself in known radial pad type bearings because it has been believed necessary to provide an accurately determined clearance between the bearing and the rotating object supported so as to allow the appropriate deflection of the bearing pads to form the hydrodynamic wedge. The requirement of close tolerances is particularly troublesome in the manufacture of gas lubricated bearings. Another problem with gas lubricated bearings is the breakdown of the fluid film at high speeds. These problems have limited the use of gas lubricated hydrodynamic bearings.
Moreover, there is still a need for a hydrodynamic radial bearing which can be used in applications where it is essential that the shaft remain centered. Currently, in applications where the shaft can not be allowed to float within a radial envelope, e.g., mechanical seals rotating element bearings are used. In rotating element type bearings, shaft centering is not a problem because the shaft is in effect maintained in solid contact with the housing. With conventional hydrodynamic bearings, however, the shaft is separated from the housing by a spacing known as the radial envelope and in operation the shaft is supported on a fluid film. Thus, because of the spacing between the shaft and the bearing surface in conventional hydrodynamic bearings, the center of the shaft tends to float or drift during operation. In mechanical seals, for example, this movement of the shaft leads to a problem known as "shaft run out" which defeats the operation of the mechanical seal. Alternatives to the commonly used tilt pad bearings have been proposed.
The focus of these attempts has been to provide simple bearing constructions which emulate the performance of more complex tilt pad bearings. For example, on pages 180-181 of Lubrication: Its Principles and Practice, Michell discusses a multiple pad bearing in which the pads are elastically pivoted on an annular member of which they form integral parts. The design shown is extremely rigid because the circumferential dimension of the neck supporting the pads is at least twice as great as the radial dimensions of the neck.
U.S. Pat. No. 2,424,028 to Haeberlein discloses a bearing member having two separate sections connected by bolts. The lower section is provided with segments and the upper section is continuous.
U.S. Pat. No. 3,107,955 to Trumpler discloses one example of a bearing having beam mounted bearing pads that displaces with a pivoting or swing-type motion about a center located in front of the pad surface. This bearing, like many prior art bearings, is based only on a two dimensional model of pad deflection. Consequently, optimum wedge formation is not achieved.
In the Hall patent, U.S. Pat. No. 2,137,487, there is shown a hydrodynamic moveable pad bearing that develops its hydrodynamic wedge by sliding of its pad along spherical surfaces. In many cases the pad sticks and the corresponding wedge cannot be developed. In the Greene Patent, U.S. Pat. No. 3,930,691, the rocking is provided by elastomers that are subject to contamination and deterioration.
U.S. Pat. No. 4,099,799 to Etsion discloses a non-unitary cantilever mounted resilient pad gas bearing. The disclosed bearing employs a pad mounted on a rectangular cantilever beam to produce a lubricating wedge between the pad face and the rotating shaft. Both thrust bearings and radial or journal bearings are disclosed.
There is shown in the Ide patent, U.S. Pat. No. 4,496,251 a pad which deflects with these web-like ligaments so that a wedge shaped film of lubricant is formed between the relatively moving parts. The use of three spaced ligaments necessarily limits flexibility and prevents simple tilting action.
U.S. Pat. No. 4,515,486 discloses hydrodynamic thrust and journal bearings comprising a number of bearing pads, each having a face member and a support member that are separated and bonded together by an elastomeric material.
U.S. Pat. No. 4,526,482 discloses hydrodynamic bearings which are primarily intended for process lubricated applications, i.e., the bearing is designed to work in a fluid. The hydrodynamic bearings are formed with a central section of the load carrying surface that is more compliant than the remainder of the bearings such that they will deflect under load and form a pressure pocket of fluid to carry high loads.
It has also been noted, in Ide U.S. Pat. No. 4,676,668, that bearing pads may be spaced from the support member by at least one leg which provides flexibility in three directions. To provide flexibility in the plane of motion, the legs are angled inward to form a conical shape with the apex of the cone or point of intersection in front of the pad surface. Each leg has a section modulus that is relatively small in the direction of desired motion to permit compensation for misalignment. These teachings are applicable to both journal and thrust bearings. While the disclosure of this patent represents a significant advance in the art, it has some shortcomings. One such shortcoming is the rigidity of the support structure and bearing pad which inhibits deformation of the pad surface. Further, the bearing construction is not unitary.
The last two patents are of particular interest because they demonstrate that despite the inherent and significant differences between thrust and journal bearings, there is some conceptual similarity between hydrodynamic journal bearings and hydrodynamic thrust bearings.
Prior art hydrodynamic bearings often suffer from fluid leakage which causes breakdown of the fluid film. In radial bearings, the leakage primarily occurs at the axial ends of the bearing pad surface. In thrust bearings, the leakage primarily occurs at the outer circumferential periphery of the pad surface as a result of centrifugal forces action on the fluid. When wedge formation is optimized, fluid leakage is minimized.
Many of today's modern turbomachines, especially those running at high speeds and low bearing loads, require the superior stability characteristics of tilt-pad journal bearings to prevent rotordynamic instabilities. Until now, the design complexity of tilt-pad bearings has precluded their use in many small, high-volume applications where cost and size are important.